dt-bindings: soc: starfive: Add StarFive syscon module

Add documentation to describe StarFive System Controller Registers.

Reviewed-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Co-developed-by: Xingyu Wu <xingyu.wu@starfivetech.com>
Signed-off-by: Xingyu Wu <xingyu.wu@starfivetech.com>
Signed-off-by: William Qiu <william.qiu@starfivetech.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>